Method of fabricating metal parts of different ductilities

ABSTRACT

Hardened metal articles such as slabs, plates or sheets or preformed metal articles which are to have regions of different ductility are subjected to heating to an austenization temperature and then to quenching of the region of greater ductility to form a start temperature lying above the gamma-alpha transformation temperature of the workpiece structure. The quenching is terminated at a stop temperature lying above the martensitic temperature and prior to any substantial transformation into ferritic or perlitic structures. The higher ductility region is then maintained prior to any substantial transformation into ferritic or perlitic structures. The higher ductility region is then maintained isothermically for condition of austenitite ferrite and/or perlite. The lower ductility structure is brought to a temperature sufficient for martensitic formation and the hardening is then carried out.

FIELD OF THE INVENTION

[0001] Our present invention relates to a method of making hardened metal parts, especially motor vehicle components, having regions of different ductility. More particularly, the invention relates to a method whereby a workpiece, such as a slab, a plate or a preformed metal part, usually of an alloy steel, is subjected to heating to an austenitization temperature and then subjected to a hardening process while being passed along a transport path and such that the end product will have at least one region of higher ductility and at least one region of lower ductility.

BACKGROUND OF THE INVENTION

[0002] It is known to produce shaped articles or vehicle components which are hardened in the die or shaping tools. These can include steering or cross bars or structural components like door impact beams, B-columns, struts or shock-absorbing portions of the chassis or vehicle body. These usually have uniform properties through the shaped component and are produced using a complete hardening of the component or by subjecting the entire component to annealing or tempering processes. These parts then generally have high strength and can retain their stability in the case of a crash.

[0003] However, it may be desirable to allow the components to be deformable in the case of a crash and thereby dissipate the crash energy as deformation energy. In certain applications in motor vehicle construction, the shaped articles are required to have certain regions of high strength and low deformability and other regions which should be of greater ductility. For example, in the case of B columns, the column foot should be relatively ductile while the upper part of the column should have greater strength and lower ductility. Apart from reinforcing the column portions which are to have reduced deformability by joining additional members to them or attaching reinforcing plates or the like, it is also known to subject a component to such heating treatment that it will have local regions of higher strength and thus lower ductility and regions of higher ductility or deformability.

[0004] Thus the German patent document DE 197 43 802 C2 describes a method of making a shaped structural component for a motor vehicle with regions of different ductility in which a starting Blab or billet prior to or after pressing is only partially heated or is, after a prior homogeneous heating, is heated further in regions in which the desired higher ductility is to be produced. The subsequent heating to generate ductile regions can result in distortion of the article.

[0005] German Patent document DE 197 23 655 A1 describes a method of partially hardening a shaped structural component whereby the starting slab is homogeneously heated in a furnace and then is hardened in a cooled pair of dies whereby partial regions of the workpiece are subjected to hardening with slow cooling in that, at these locations in the tools, recesses or thermally insulating inserts are arranged or inductive heating is effected at these regions. The purpose of this process is to enable a workpiece to be machined additionally in the partially nonhardened regions, for example by boring. The method of DE 197 23 655 A1 however has problems in the case of hot-forming processes since the shaping cannot occur in the regions in which recesses are provided in the tools and in which greater ductility is to be provided by preventing or limiting the hardening. As a consequence breakage can occur. The inductive hardening is only possible for the finally shaped parts and requires a separate process step. As a consequence the subsequent inductive hardening is expensive and can involve the danger of distortion.

[0006] European Patent EP 0 816 520 B1 describes a shaped article and a method for creating defined strength and hardness characteristics thereof over its length whereby the article after shaping is inductively heated and then quenched to produce hardened regions.

[0007] DE 200 14 361 U1 describes a B column which also has regions of different strength. The production of the B column is effected by a hot forming process in which a steel blank or preformed elongated section is austenitized in a furnace and then shaped and hardened in a cooled die. In the furnace, large-area regions of the workpiece can be shielded against the effect of the temperature by insulation so that in the shielded regions the austenitization temperature is not reached and as a consequence in the workpiece there is no martensitic structure upon hardening.

[0008] Alternatively, it is proposed to subject the steel section completely to austenitization and during the transport to bring a region to a temperature significantly below the austenitization temperature, for example by blowing onto this region for a cooling at a limited or slow rate within the hardening tool. In the hardening tool, therefore, no pure martensitic structure arises but rather a mixed structure with clear ferrite/bainite structures is produced which has ductile characteristics.

[0009] This process has several problems in its practical application in mass production. The use of insulation of shielding in the furnace is itself an expensive operation since in each cycle individual parts must be insulated separately. The insulation must be provided in a preparation stage and increases the duration of the heating process and the insulation, in the case of re-use must be heated up. This makes mass production expensive. A cooling which is intended not to be too sharp is difficult to control, especially when it is intended to bring the temperature to a point significantly below the austenitization temperature and is a problem for mass production systems. The products which are produced may have variable properties as a consequence.

OBJECTS OF THE INVENTION

[0010] It is, therefore, the principal object of the present invention to provide a method of making metal products, especially vehicle structural components, with at least two regions of different ductility, which is suitable for mass production.

[0011] Another object is to provide a method for the purposes described which avoids the problems hitherto encountered in fabricating steel articles with regions of different ductivity.

SUMMARY OF THE INVENTION

[0012] These objects are achieved, in accordance with the invention, in a method of producing a hardened metal article with at least two regions of different ductivity including at least one first region with higher ductivity and at least one second region of lower ductivity. The method comprises the steps of:

[0013] (a) heating a metal slab or preformed metal part constituting a workpiece to an austenitization temperature;

[0014] (b) thereafter transporting the workpiece over a transport path and during transport of the workpiece along the path subjecting the first region to cooling by:

[0015] (b1) quenching the first region from a predetermined cooling start temperature (T_(start)) lying above a γ-α transformation temperature of the workpiece,

[0016] (b2) terminating the quenching when a predetermined cooling stop temperature (T_(stop)) is reached which lies above the martensitic start temperature and prior to any transformation into ferritic or perlitic structures or prior to any but a slight transformation into ferritic or perlitic structures, and

[0017] (b3) then maintaining the workpiece approximately under an isothermal condition for conversion of austenite in the structure of the workpiece to at least one structure selected from the group consisting of ferrite and perlite;

[0018] (c) during step (b) maintaining the second region at a hardening temperature (T_(H)) at least sufficient for martensite formation in the second region; and

[0019] (d) then effecting a hardening process for the workpiece during transport thereof along the path.

[0020] In step (a), therefore, the metal slab or preformed metal parts is brought in a heating device to a defined austenitization temperature for a predetermined austentitization time and thus is homogeneously heated to a temperature which can correspond to the cooling start temperature.

[0021] By contrast with a continuous cooling of the first regions at a low cooling rate, the invention provides in its first step for a rapid quenching of the first regions to a cooling stop temperature or transformation temperature and then a substantially isothermal transformation into a ferritic/perlitic structure. This has the advantage that an exact setting of the transformation temperature parameter and the retention time parameter for the ferritic/perlitic structure component can be readily set and controlled and thus that the mechanical characteristics are controllable in a highly reliable manner. It is also of advantage that the parallel processes for creating the ductile characteristics of the first regions and the process for creating the low ductility high strength second regions have identical process commencement and the same terminations and hence the same process times. The method can thus be integrated in a problem-free manner in already existing hot-forming processes.

[0022] In an alternative, the quenching step can commence at a higher cooling speed which is greater than the critical cooling speed, i.e. the cooling speed at which a ferritic/perlitic structure is formed and which can be halted at a precisely determined temperature. This temperature is so chosen that it is a maximum for the ferritic/perlitic transformation at the highest possible rate and simultaneously is a compromise. At lower temperatures, the transformation of the austenite is greater but the increasing diffusion inertia of the carbon atoms delays the process. In contrast thereto the diffusion of the carbon atoms is significantly greater at higher temperatures but the transformation of the austenite is much less. The duration of the retention time required for structure transformation also has the direct influence on the amount of the remaining residual austenite content in the first regions.

[0023] Since this retention time cannot optionally be increased in the case of mass production and the hardening temperature for the second regions optionally lowered, a fairly precise agreement between the different cooling processes to which a workpiece must be subject is required. The optimization of the temperatures and retention times ensures ductile and high strength regions in a single structural component.

[0024] During the isothermal transformation in the first regions, the second regions are predominantly or completely maintained in the austenitic range. As a result it is of special advantage to match the transformation time span with the austenitization temperature selected in the heating furnace such that the hardening temperature for the second regions over the transformation time is less than the heating temperature in the furnace.

[0025] It is especially advantageous and an optimal match for the hardening temperature to be so high that a martensite formation in this region occurs during the hardening process. Preferably an excessive temperature drop in the second region can be counteracted by a supply of heat thereto during the transformation of the first region. It can be sufficient, in this case, to avoid radiation loss from the second region or to minimize radiation loss, for example by reflecting radiation back onto the second regions.

[0026] In order to ensure that the rapid cooling process and the isothermic stage are reproducible, the first regions are cooled, in accordance with the invention, with a cooling medium dispensed from nozzles conforming to the geometry of the first regions of the workpiece. The cooling medium is preferably an air stream.

[0027] The hardening process can be carried out in any optional hardening device, for example, in a quenching vessel. It is however especially advantageous to effect the quenching using a cooled tool, for example, a shaping die in conjunction with a shaping operation. This mode of operation has been found to be especially effective when the process is part of a hot-forming process. In that case the hardening step, which involves quenching below the martensitic starting temperature or forming martensite in the austenitic structure of the second regions, is effected in contact with the cooled die. Additional steps such as tempering or annealing can follow. The result is a continuous rather than an abrupt transition from the more ductile structure to the harder structure between the first and second regions.

[0028] In addition to air nozzles conforming to the geometry of the workpiece for local cooling of the first regions, it is advantageous to shield the processes in the two regions from one another, for example by a partition in the form of a sheet or plate. This permits the transition from first regions of high ductility to regions with higher strength to be established with precision.

[0029] A precipitous transition from ductile to the high strength over a small transition region can thus be obtained if desired or a transition region which is wide and gradual can be created with the material characteristics running from ductile to high strength or vice versa depending upon the desire of the operator.

[0030] The method is particularly suitable for use with steel alloys containing manganese and boron. With such steels the critical cooling speed, i.e. the cooling speed which a martensitic structure arises is significantly reduced. The boron addition results, during the cooling of steel in a delay of the transformation into softer structural types like ferrite and perlite starting from the austenitic range. This means that slower cooling speeds can produce a hardening in the material like that which can be achieved with a continuous air stream. These steel types have been hardened in according with the German patent document DE 200 14 361 U1 using an air stream over the entire structure of the workpiece and will not yield ductile regions.

[0031] The invention is preferably applied to a slab of a steel alloy having, in weight percent, carbon between 0.18% and 0.3%, silicon between 0.1% and 0.7%, manganese between 1.0% and 2.5%, phosphorus to a maximum of 0.025%, chromium from 0.1% to 0.8%, molybdenum between 0.1% and 0.5%, sulfur to a maximum of 0.01%, titanium between 0.02% and 0.05%, boron between 0.002% and 0.005% and aluminum between 0,01% and 0.06%, the balance being certain unavoidable smelting impurities. Although not mandatory the steel alloy can have a niobium content (Nb) between 0.03% and 0.05% to minimize intercrystalline corrosion and resistance to heat.

[0032] The method of the invention with the described interrupted quenching step and the isothermal retention at a temperature above the martensitic start temperature in the case of boron and manganese-containing steel ensures ferrite/perlite transformation for a softer structure in the first regions of the workpiece. Because of the presence of boron, it is possible to provide a reduced hardening temperature in the second regions so that during the retention time a harder structure is ensured with the requisite higher strength.

BRIEF DESCRIPTION OF THE DRAWING

[0033] The above and other objects, features, and advantages will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

[0034]FIG. 1 is a schematic diagram of the method of the invention;

[0035]FIG. 2 is a graph of temperature vs. time illustrating the transformation start and end points and the times at which they occur; and

[0036]FIG. 3 is a positive view illustrating aspects of the invention and in particular the cooling of the workpiece with a partition or shielding between process zones.

SPECIFIC DESCRIPTION

[0037]FIG. 1 shows the process sequence in the production of structural components for a motor vehicle having regions of different ductility. The fabrication line comprises a heating unit 1 or furnace in which the slab, plate or sheet 2, or a preformed component, is homogeneously heated over a certain austenitization time t_(a) to a predetermined austenitization temperature T_(A). On the transport path to a hardening unit 3, for example, a reshaping die and press, in which the slab is then subjected to shaping under uniform cooling, the process is subdivided into two process stages P1 and P2 in which the local processing of different regions of the workpiece enables the creation of different deformation properties in the workpiece which will remain in the finished product.

[0038] Between the heating stage 1 and the hardening stage 3, in a process line P1 in which the more ductile regions are to be formed in the first regions of the finished product, there is, for example, a heating bed wherein, for instance, in case the intrinsic heat of the component is not sufficient, hot air is blown therein. The zone for maintaining the austenitic region 6 of the second or less ductile part in the second process line P2 is preferably provided with an additional heating device 7, for example, having induction coils. The radiant heat from this second part of the workpiece can also be reflected back onto the workpiece by means of a mirror or other reflective unit 8.

[0039] When the component is a previously fabricated component, for example, a B-column, the column after heating in the furnace is displaced with its longitudinal axis traverse to the transport direction on a conveyer belt which is represented by the paths P1 and P2 between the furnace 1 and the die 3. The column foot is initially rapidly cooled (quenched) at 4 and then over the stretch 5 held isothermally while the structure of the workpiece in the upper column part by transport through the zone 6 is maintained in the austenitic range. Then, the component is subjected to hardening and shaping in the cooled dies 3.

[0040] The temperature course of the two partial process lines P1 and P2 has been represented in FIG. 2. Starting from the common austenitization temperature T_(A), the first regions, which are to be softer in the finished product and thus to have a more ducted structure, are brought from the cooling start temperature (_(Tstart)), which here corresponds to the austenitization temperature, at the time t₁ with a cooling or quenching rate of 100 to 200 k/s to the cooling stop temperature (_(Tstop)) or a transformation temperature of 400° C. to 800° C. at the time t₂. The first regions are then subjected to isothermal transformation approximately at this temperature to the time t₃. During this period, the second regions which are to have a structure with reduced ductility in the final product are maintained in the austenitic range until the transformation of the structure of the first regions has been concluded or is nearly concluded. At the time T₃, the hardening process occurs in which both regions are quenched. The first regions are quenched from the temperature T_(stop) while the second regions are quenched from the hardening temperature T_(H).

[0041]FIG. 3 shows, in a perspective view, a shaped component 9 with a ductal region 10, referred to herein as the first region, and a low ductility high strength region 11, referred to as the second region. The transport direction is represented by the arrow A.

[0042] The two regions 10 and 11 can be separated by a sheet or plate 12 forming a partition between the process zones of the workpiece as it passes along the transport path through the process stages P1 and P2. The partition 12 matches the shape of the workpiece 9, The region 10 which is to be more ductile in the finished product, is juxtaposed above and below with nozzles 13, 13 a, 13 b, 13 c, 13 d and 13 e which are shaped to conform to the contour of the workpiece and inform which air is directed at the workpiece to effect the quenching or rapid cooling defined in FIGS. 1 and 2. The cooling medium may be air. During this cooling part 11 which is to be less ductile and of higher strength in the finished product is not subjected to cooling and indeed is protected by the partition 12 from cooling.

[0043] The resulting part, when finally hardened, has regions with two different structures and ductility and the corresponding different mechanical properties. The particular temperatures and times used can be matched to the different alloying elements and compositions employed and the method has been found to be applicable to components having large regions of high ductility while avoiding problems hitherto encountered like distortion and the requirements for extra steps.

[0044] A suitable composition in accordance with the invention and provided as an example is a manganese-boron steel alloy of the following composition (in weight %):

[0045] carbon (C) 0.18% to 0.3%

[0046] silicon (Si) 0.1% to 0.7%

[0047] manganese (Mn) 1.0% to 2.50%

[0048] phosphorus (P) maximum 0.025%

[0049] chromium (Cr) 0.1% to 0.8%

[0050] molybdenum (Mo) 0.1% to 0.5%

[0051] sulfur (S) maximum 0.01%

[0052] titanium (Ti) 0.02% to 0.05%

[0053] boron (B) 0.002% to 0.005%

[0054] aluminum (Al) 0.01% to 0.06%. 

We claim:
 1. A method of producing a hardened metal article with at least two regions of different ductility including at least one first region with higher ductility and at least one second region of lower ductility, comprising the steps of: (a) heating a metal slab or preformed metal part constituting a workpiece to an austenitization temperature; (b) thereafter transporting said workpiece over a transport path and during transport of the workpiece along said path subjecting said first region to cooling by: (b1) quenching said first region from a predetermined cooling start temperature (_(Tstart)) lying above a γ-α transformation temperature of the workpiece, (b2) terminating the quenching when a predetermined cooling stop temperature (_(Tstop)) is reached which lies above the martensitic start temperature and prior to any transformation into ferritic or perlitic structures or prior to any but a slight transformation into ferritic or perlitic structures, and (b3) then maintaining the workpiece approximately under an isothermal condition for conversion of austentite in the structure of the workpiece to at least one structure selected from the group consisting of ferrite and perlite; (c) during step (b) maintaining said second region at a hardening temperature (T_(H)) at least sufficient for martensite formation in said second region; and (d) then effecting a hardening process for said workpiece during transport thereof along said path.
 2. The method defined in claim 1 wherein said second region is brought to said hardening temperature (T_(H)) during step (b3) which is less than the heating temperature in step (a).
 3. The method defined in claim 1 wherein said second region is subjected to heating to maintain an austentite structure therein.
 4. The method defined in claim 1 wherein heating radiated from said second region is captured by a reflecting mirror and reflected back to said second region.
 5. The method defined in claim 1 wherein said first region is cooled in step (b1) by a nozzle directing a cooling medium onto said first region and matched to the geometry of the workpiece.
 6. The method defined in claims 5 wherein said cooling medium is air.
 7. The method according to claim 1 wherein the hardening process is carried out in a cooled reshaping tool in the course of a hot forming of the workpiece.
 8. The method defined in claim 1 wherein said first and second regions are subjected to processes separated by a partition from one another.
 9. The method defined in claim 1 wherein the workpiece is composed of a steel alloy containing manganese and baron components.
 10. The method defined in claim 9 wherein the workpiece is comprised of a steel alloy consisting, in weight percent, essentially of: carbon (C) 0.18% to 0.3% silicon (Si) 0.1% to 0.7% manganese (Mn) 1.0% to 2.50% phosphorus (P) maximum 0.025% chromium (Cr) 0.1% to 0.8% molybdenum (Mo) 0.1% to 0.5% sulfur (S) maximum 0.01% titanium (Ti) 0.02% to 0.05% boron (B) 0.002% to 0.005% aluminum (Al) 0.01% to 0.06% balance iron and unavoidable smelting impurities. 